Recently, a polysilicon film of high carrier mobility has come to be used as a channel layer of a thin film transistor used, for example, for a liquid crystal display. The polysilicon film, used as the channel layer of the thin film transistor, is routinely manufactured by heat-treating amorphous polysilicon on a glass substrate by illuminating laser light thereon. The method for heat-treating an object by illuminating laser light thereon is termed laser annealing and an apparatus for carrying out the laser annealing is termed a laser annealing apparatus.
In producing a polysilicon film, laser annealing needs to be carried out with a laser beam having uniform energy strength along the beam diameter, in order to prevent thin film transistor characteristics from becoming worsened.
However, with the collimated laser beam, formed, for example, by a collimator, the energy strength distribution within the beam diameter is of a Gaussian distribution.
That is, a usual collimated laser beam has a high strength in the center of the beam diameter, while having a low strength in a rim part of the laser beam. Thus, with the laser annealing device, it is necessary to form a laser light beam with a uniform strength distribution from the center up to the rim of the spot from the laser light beam having the Gaussian strength distribution within the beam diameter and to carry out heat treatment using the so formed laser light beam. The routine practice with the laser annealing device, the laser light radiated from a laser oscillator is collimated, for example, by a collimator, a plural number of laser beams are then formed by a light splitting means, such as a fly-eye lens, and the so formed laser beams are again combined together to provide for uniform strength distribution of the laser light illuminating area on a substrate.
Among known laser oscillators, there is a solid laser formed of a transparent material, such as crystal or glass, excluding a semiconductor, as a matrix material. The solid laser excites a solid laser material, composed of a matrix material and rare earth ions or transition metal ions doped therein, with light, to radiate a laser light beam.
With the solid laser, the radiated laser light is stable and has a long useful life. Hence, with use of the solid laser as a laser light source for the laser annealing device, it may be contemplated that the problem of instabilities, otherwise produced with the use of the excimer laser used as a customary light source for the laser annealing device, may be overcome.
The laser light radiated from the solid laser has high interfering properties as compared to the laser light radiated from the excimer laser. Thus, if the solid laser is used as the laser light source, and the laser light beams, split by the fly eye lens, are synthesized, the light beams interfere with one another. In the laser light, which has undergone the interference, interference fringes are produced in an illuminated light spot, such that it is not possible to provide for uniform strength distribution in the beam diameter.
For overcoming this drawback, the present Assignee has proposed, in the specification and drawings of Japanese Patent Application 2001-374922, a laser annealing device employing a light splitting optical means which may be used in place of the fly eye lens for splitting a sole laser light beam into plural laser light beams not interfering with one another.
The laser annealing apparatus, proposed in the aforementioned patent application is briefly explained. FIG. 1 schematically shows the annealing apparatus proposed in the aforementioned patent application.
In the laser annealing apparatus 100, shown in FIG. 1, a laser light beam L120 is radiated from a laser light source 101. The laser light beam L120, radiated from the laser light source 101, is collimated by a collimator 102 to fall on a light splitting unit 103.
The light splitting unit 103 includes first and second beam splitters 104, 105, and a reflecting mirror 106. The light splitting surface of the first beam splitter (BS) 104, the light splitting surface of the second beam splitter (BS) 105, and the light reflecting surface of a reflecting mirror 106 are all arranged parallel to one another.
The laser light beam L120, collimated by the collimator 102, is incident on the first BS 104. The first BS 104 separates the laser light beam L120 into a transmitted light beam (termed a laser light beam L121) and a reflected light beam (termed a laser light beam L122). The first BS 104 splits the light into the transmitted light beam and the reflected light beam at an intensity ratio of 1:1.
The laser light beam L121 falls on the second BS 105, which second BS 105 further splits the incident light L121 into a transmitted light beam (termed a laser light beam L123) and a reflected light beam (termed a laser light beam L124). The second BS 105 splits the light into the transmitted light beam and the reflected light beam at an intensity ratio of 1:1.
The laser light beam 123 falls on a first convex lens 107. The laser light beam L 124 is reflected by a reflective mirror 106 to fall on a second convex lens 108.
On the other hand, the laser light beam L122 is reflected by a reflecting mirror 106 to fall on the second BS 105. The second BS 105 further splits the incident laser light beam L122 into a transmitted light beam (termed a laser light beam L125) and a reflected light beam (termed a laser light beam L126). The second BS 105 splits the light into the transmitted light beam and the reflected light beam at an intensity ratio of 1:1.
The laser light beam 125 falls on a third convex lens 109. The laser light beam L126 is reflected by the reflective mirror 106 to fall on a fourth convex lens 110.
The four laser light beams L123, L124, L125 and L126, thus generated, are parallel to one another and are each of an intensity one-fourth that of the pre-splitting laser light beam L120.
The laser light beams L123 to L126 are once condensed by the first to fourth convex lenses 107 to 110, respectively, so as to be then incident on a condenser lens 111, which condenser lens 111 then illuminates the laser light beams L123 to L126 in a preset range on a substrate 112.
With the above-described laser annealing apparatus 100, the spacing t between the light splitting surface of the first BS 104 and that of the second BS 105 and a spacing t between the light splitting surface of the first BS 104 and the reflecting surface of the reflecting mirror 106 are set so as to satisfy the following equation 1:t>L/(2ncos θ)  (1)where L is the coherence length of the laser light beam radiated from the laser light source 101, n is the refractive index of a medium between the light separating surfaces and a medium between the light separating surfaces and the reflecting mirror, and θ is the angle of incidence of the incident light beam on the light splitting surfaces.
Thus, even though the laser light beams L123 to L126 are radiated from the same laser light source 101, the totality of the light paths are longer than the coherence length, such that none of these light beams interfere with one another. Hence, with the laser annealing apparatus 100, the preset range of the substrate 112 can be illuminated with the uniform intensity, without forming interference fringes, and hence the object to be illuminated may be illuminated uniformly in its entirety.
Meanwhile, with the above-described laser annealing device 100, the ratio of the volume of light transmission and that of light reflection by the BS 104 and the BS 105 is ideally 1:1. However, due to manufacturing tolerances, the ratio of the volume of light transmission and light reflection actually is not 1:1. On the other hand, it is desirable that the reflectance of the reflective mirror 106 is ideally 100%. However, due to manufacturing tolerances, the reflectance actually is lower than 100%. Consequently, the laser light beams L123 to L126, radiated from the light splitting unit 103, are actually not uniform in intensity, even though it is ideally desirable for these light beams to be uniform in intensity.
FIG. 2 depicts a graph showing the values of light intensity of the laser light beams L123 to L126 in case the ratio of the reflectance and transmittivity of the BS 104 and the BS 105 suffers an error of 2% and the reflectance of the reflective mirror 106 is 99%. FIG. 2 shows the proportions of the intensity of the respective laser light beams L124 to L126, with the intensity of the laser light beam L123 as a reference.
It may be seen from FIG. 2 that, in case the four laser light beams L124 to L126 are generated under the above conditions, there results the difference in the light intensity of the order of 17%. The reason that this large difference in light intensity is produced despite the small values of the errors of the BS 104, BS 105 and the reflective mirror 106 is that the components of manufacturing tolerances are accumulated as a result of multiple reflection.